Marshall JVM410h - blown fuse FS6 - no sound - bad solder joints.

[spunko]

The amp was good one night, next morning no sound at all.
I took the chasis out and noticed FS6 fuse was burnt. I replaced the fuse, and using a bulb limiter turn on the amp. No blown fuse this time, then tested without the bulb limiter and it worked fine.

I decided to put some stress and leave it ON for 5 or 6 hours, and checked it several times every hour. At the end of the night the amp was fine. Tubes where fine (I always check for red plating), sound was crystal clear, everything was fine.

So today I wanted to do the same, but as soon as I hit the standby switch I heard some pops and hum for the first time since I have this amp, and immediately noticed that V3 EL34 was red plating. I swapped V3 and V4 to see if the symptom followed the tube, but again V3 was red plating, so the problem is in that socket.

I flipped the chasis again and took out the tube from V3, and reflowed the solder on R40 screen resistor, installed the tube and tested again. This time everything was fine, no red plate, no blown fuse, bulb limiter was fine, but I heard some pops and cracks, so I taped the tubes a bit and V3 was super sensitive, and when I hit it a bit harder with my finger, the amp started to hum badly, so I turn it off.

I installed a new tube on V3, and now the bulb limiter goes from dim to very bright in a matter of 5 seconds, it hums and also the red LED from the power swicth starts to flick and then starts to go out little by little (just the LED). Tested the original tube, same result, tested another brand new tube and the same 3 symptoms.
If I test the amp without V3, everything is good, no hum, clear sound, bulb limiter fine. If I install the tube all the problems above appear.

When changing the tube again, I noticed that three solder joints from V3 were moving along with the tube, so I thought that was the problem and reflowed those bad spots, tested the amp again, and the same 3 symptoms. Take out the tube and the amp is fine.

What else could be wrong? or where do I have to look now?

https://el34world.com/charts/Schematics/files/Marshall/Marshall_jvm410_sch.pdf

[PRR]

> I noticed that three solder joints from V3 were moving along with the tube, so I thought that was the problem and reflowed those bad spots

How can this be? Isn't this a printed circuit board amp? If the solder blobs "move", they have probably torn-loose from the board. Electrical connection hit-or-miss. Some "misses" let the tube run-away, red-plate.

This may be a heroic repair. Maybe small jumpers, maybe hacking huge sections of board away and rigging new sockets and wires.

[spunko]

So I inspected again the board and noticed some more bad solder joints, and a broken trace for the negative bias on V3.

I really don't know what to think about this amp anymore. This is an expensive amp, and it shouldn't come this way, and is new btw.

I have reflowed all those bad joints and now the amps is good and working.

[PRR]

It is a $3,000 amplifier!!

You can't send it back? (I can't find any warranty info on the Marshall site.)

[shooter]

and is new btw.

Sadly you're not the 1st.
since you opened n worked on it, it probably voided any warranty.
I've told 2 musicians that had "new" amps that were broke, take it back, tell them the PCB is defective and have the store call me if they have don't believe you.  both got their money back, but I didn't do any repairs, just looked
one was a re-issue Fender, the other a re-issue B52


quality died at the 21st century mark

[spunko]

Unfortunately, here in South America there is no customer support.
Also the store told the client they wouldn't take the amp back, nor service it. It has several months out of the store, and little hours of play, but they just don't care.
I really don't know about laws in here, I bet he could do something, but it will take lawyers, time, and money, and at the end maybe he won't get his money back. You know, everything here moves with money under the table.

I told him to contact Marshall directly explaining the situation, he might get help and they force the store to give him the warranty, but I think he didn't reach them.

Either way, the amp is ready, but I feel a little worried that any other solder joint fails elsewhere.